Messages

731

General Mach Discussion / Re: Preparation Positioning, bug?

« on: April 06, 2008, 01:46:01 PM »

I understand what you are saying, and in theory you are right. However, I have no idea what mathmatics are involved in keeping track of where Mach3 is, and , more importantly, when it is implemented. If you press the Stop or Feed Hold button whilst a line is being processed, how do you know if the full details of the new position has been logged. Yes I appreciate if it is a non-move, the positional data should be the same - but the logic might post the data in a temporary file at the start of a line. It might post the new position at the end of the move and if it hasn't moved pull the data back from the temporary file. When you stop half way through it might prevent that happening, and if you start on another non-move it could get totally confused.

I must admit, I don't know the exact answer, and perhaps someone could look at it, or explain if there is a more reliable way to do what you want.

As I say, Mach3 does not always start again from where you are. It steps back and tries to gather breath - presumably to find it's position, and then carries on.

We'll see if any of the higher ups can have a crack at this one.

732

General Mach Discussion / Re: Video Window plugin Issues

« on: April 06, 2008, 01:22:15 PM »

I'll let the lads on the hill answer that one - only to say there are one or two issues with the USB cameras.

The main problem seems to be with those cameras that plug in and use the Windows drivers, rather than have their own

733

General Mach Discussion / Re: How to wire steppers???

« on: April 06, 2008, 12:48:41 PM »

Yes Ian - I,ve thought of that too.

I can set the output current on the drives. If this is current limiting to the drives, then I would think that I can put them in parrallel, and get the benefit from using both coils.

On the quick experiment I did on parrallel, it would seem that there is more power than in single coil config, and I get a little more speed, which gives me a bit more choice.

I will get on to the supplier on Monday and find out. If I can use them in parrallel without problems I think I will

734

General Mach Discussion / Re: "Holding" vs "Running" torque

« on: April 06, 2008, 12:36:06 PM »

I always thought that the reason for using stepper motors was
1. To move by a number of steps to another position
2. When at that position - HOLD IT THERE
hence the holding torque is an important figure, if not the most important torque for this type of motor.

I would also think that it is an easier figure to both calculate, and verify by experiment, since it is stationary.

When we complain of loosing steps, we all assume that we loose them whilst moving - is it possible we are loosing them whilst the motor is stationary???

I think that a 70% figure sounds reasonable.

735

General Mach Discussion / Re: How to wire steppers???

« on: April 06, 2008, 05:54:20 AM »

Thanks for the replies - I'll bear in mind the Gecko 203V card if any of my present ones fry. I can put the motor(s) back to parrallel then FLYYYYYYYY.

736

General Mach Discussion / Re: Stepper wiring scenarios

« on: April 06, 2008, 05:39:19 AM »

I started all this off - and we are now on "Hot  Thread" status.

Basically with 8 wire motors (assuming you are going to wire these to 4 output drivers) there are three ways.

My problem was my motors would not accelerate up to anything like full speed, using the drivers that I had. The motors were 7.5v, rated at 2.5 amps per winding (there are four windings A.B.C.D) . The resistance for each winding was 3 ohms. My cards are rated at 30volts (actually I use 24 volts) and 2.5 amps max.

I first connected these in series - A to C - and B to D. Electrical circuits in series add the resistance so the motor became a 2 pole motor with 6 ohms resistance on each winding.

Power from an electric motor is dependent on the amount of current going through it, and by connecting the windings in series I had doubled the resistance, which cut the current (and therefore the pwoer) by half. There is one way of rectifying that problem - increase the voltage which will force more current through, giving more power - but unfortunately my drives were limited to 30 volts, so I was not going to get much increase there.

My second effort was to rewire the motors - this time in parrallel - A and C together, B and D together. In parrallel, resistance is, for the sake f this post, halved.  The same 24 volts would force additional current through the windings, giving me more power - in theory 4 times the power.

It did work, and I got the motors spining at up to 30 ins per minutes ( a massive increase over the 4 ins per minute I had before).

I had measured the current drawn whilst doing this - and this is where I went wrong. Because these are stepper motor driven electronically by chopper circuits, the current shown on a meter is not an accurate reflection of the current drawn by the windings. The very conservative 0.9 amps reading I got is apparently way off the mark. The agreement is I am lucky still to have my driver board in one piece, or at least not fried. I am not sure but they may by current limiting.

To sustain that performance, I would need a set of drivers able to carry twice the current.

The compromise was to only use two of the windings -  A and B. Wired singly they have 3 ohms resistance, which means they carry 2.5 amps. This still gives me the OMPH I am looking for at 24 volts, and they succesfully run the axis at 16 ins per minute. I have dropped this to 12 for the sake of accuracy (and so I do not drop steps) - sinceI do not need tremendous speed on my lathe. (18"" bed).

I now have 3 times the speed for G0 movements, still have more power, at a net cost of zero.

737

General Mach Discussion / Re: Preparation Positioning, bug?

« on: April 06, 2008, 05:01:55 AM »

It says, quite clearly in the notes, that using "Run from Hrere" Mach may step back a few lines to pick up.

I do not recommend "Run from Here" and is is, I would have thought, bad practise to use it in normal work. I think it is designed for, and should be used, only when something unforseen has stopped the program. You E stop, or whatever, rectify the work, and (instead of running right from the beginning again) try to carry on (because it is quicker to go forward than go back.

The problem is (as you have found out) POSITION.

Unless you have some independent system, reading back into Mach3, then position is calculated on the move, from your base position - either when you home, or when you set up your work. From this start position Mach 3 adds or subtracts distances, or offsets as it moves.
If you step backwards through the program it can calculate that as well. But if you jump Mach has to find its position. It does that by trying to find its last positional move and working from there, but if offsets and other similar types of "distractions" are there it doesn't always get it right.

If you are going to stop at that place on a regular basis, I would put in a stop (M1). If you are only going to do part of the drilling program and then want to advance, then put in a positional move G0 to a safe tool spot before the G80 and "Run from There"

738

General Mach Discussion / Re: microsteps should not be included in resolution calculations

« on: April 06, 2008, 04:37:26 AM »

To put it all together -

You can guarantee that with a stepper motor any STEP will be within 3 to 5% accuracy when it stops ( and it is only on that step) - so if you have 200 step per rev, the accuracy of the stop is 5/20,000 of a full rev or 0.09 of a degree if my maths is right.

The accuracy of the micro steps cannot be guaranteed, but even if they were twice as large since there are 8 to make up a step, it must be still more accurate that a full step.

The only time it comes to bite you is if you are relying on this in your computation of steps per unit.

Mine is geared down, and I need 48,000 steps per inch. So I need 48 steps per thousanth of an inch so I can get very accurate (if I can manage 1/2 a thou I will be happy, so I have 24 steps to play with).
If I geared up my axis drive for speed - say direct motors and much more pitch on the leadscrew - took out the 3 to 1 reduction and altered the leadscrew to say 1/2 pitch (a bit excessive, but you get the idea) then I would be down to 8,000 steps per inch - only 8 steps per thousanth of an inch.

Still enough for me, you might say, but in the quest for speed (some talk of 100 inches per minute) these ratios might increase until you were is a position when microsteps start coming into the calculations of standard units of mm or inch. If, say, you got down to 5 microsteps per thousanth then you cannot guarantee the accuracy as you can with a full step.

A lot of time spent discussing a very small difference - since it is non-cumulative - but neither can it be calculated.

It all depends on what you want your machine for - if you are working in wood clearly any accuracy you decide on does not really matter, because wood expands and contracts dependent on humidity. Some plastics do the same with temperature.
I work mainly with steel and would like to be able to guarantee 1/2 a thou on my parts (I can't measure any better anyway) - so I am  on 48,000 steps per inch - my movement (if you followed the other thread) I have settled at 12 inches per minute, which is comfortably within the capablilities of my motors - without taking too long to move about.

It all depends what you want - accuracy or speed.

739

General Mach Discussion / Re: How to wire steppers???

« on: April 04, 2008, 01:47:10 PM »

Right -

I rewired all three steppers - to two single coils, refitted them all.

Great - I can now get 16 in per minute on all axis. This is fabulous - 4 times what I was getting on bipolar series wiring.

This for me is fine. My mill table is only 12" by an effective 10" so I can move any where in 45 secs. In lathe mode the length of the leadscrew is 18" so I can move from end to end in about a minute.

Thanks for the other suggestions, but altering the gearing, which are timing belts is a bit difficult, because I have glued the gear onto the motor shaft, although this would increase the speed. The Z axis I would leave because I want the torque when drilling and it can be up and down in seconds.

The voltage increase is also a bit difficult, because I use a pair of traction batteries to provide a very steady 24 volts.

That leaves the leadscrew, which I will replace eventually, to cut backlash, and I will certainly consider increasing the pitch of that.

If there are any more suggestions thanks - and thanks for the help etc already - a fourfold increase for no expenditure is excellent.

Jim.



 

740

General Mach Discussion / Re: How to wire steppers???

« on: April 04, 2008, 07:49:35 AM »

Ian

WWWWHHHHJEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE. Not quite the sound of my motor. I'm sorry, I was quick off the mark, having read your post just now - particularly the bit about the meter reading.

No harm done, it would seem. I wired in parrallel and got the motor up to 30 ins per min. I ran through 1/8 to 1/2 steps, but I have ended of 1/8 phase again. I settled on a modest 15 " per min for reliability. This is still nearly four times faster. Even at 20" per min it was good, with only the occiasional stoppage.

I will take heed of your warnings however, and disconnect two of the coils, C and D, and see if it makes any difference. It certainly did so far.

Meter reading for parrallel - 0.7 amps. I must admit I thought this was far too low.